National Republican Campaign Committee has launched internal investigation into ‘cyber intrusion’ and notified the FBI

WASHINGTON—The campaign arm for Republican congressional candidates fell victim to a cyberattack in April by an unidentified intruder, people familiar with the matter said on Tuesday, fueling concerns that the 2018 election campaign may have been more seriously targeted by hackers than previously known.

The identity of the hacker wasn’t clear, but one person familiar with the investigation said it was believed the attack was from a foreign operator. Other people said they couldn’t attribute the source of the hack but that...